The present invention relates to a method and circuit for the reconstitution, by filtering, of an analog signal, from a pseudo-analog signal obtained by a double analog to digital, digital to analog conversion of the analog signal along with processing of the digital signal.
In greater particularity, the subject invention relates to the filtering of a pseudo-analog signal in which the analog variation of amplitude as a function of time are represented by incremental level variations.
This problem of filtering is very important in relation to the restitution of graphic or iconographic documents which have been previously digitally encoded.
In the digital transmission of photographs or documents requiring the presence of half-tones, every shade of grey in black and white (or of luminence in color) is represented by a level. If a faithful restitution of the document is desired, it is necessary to increase the number of points (pixels) to be quantified, and to provide a large number of levels from black to white; two hundred and fifty-six, for example. In this case, a low-pass filter with a relatively short time constant will suffice to filter the signal such that it is identical to the original signal.
This solution, even if various compression codes are used, has the drawback of requiring a large number of binary data. In the case of an A4/2 format, for example, taking 4 points per mm, there would be 504,000 pixels and 4,032,000 binary bits.
This large quantity of binary bits makes it necessary either to use high speed transmission lines in order to transmit a document quickly, or to use complex compression codes to reduce the number of binary bits to be transmitted to a lower number. This technique calls for complex and expensive equipment for both the encoding and decoding functions.
Other solutions consist in the digital addition of an electronic noise signal to the signal, thereby making it possible to linearize the decoded signal and make the transitions less abrupt. This solution, likewise, is complex and expensive, and does not provide a really satisfactory quality restitution.